1. Field of the Invention
The present invention relates to a variable dispersion compensator, which compensates for dispersion that develops in an optical signal due to its propagation through an optical transmission line such as an optical fiber transmission line, and an optical transmission system in which it is provided.
2. Description of the Related Art
In an optical transmission system in which optical signals are propagated by optical fiber transmission lines and the like, dispersion accumulates in optical components of each frequency (each wavelength) that is included in an optical signal through the dispersion that exists in an optical fiber. In this regard, it is desired that the amount of dispersion in an optical transmission system be reduced in the frequency band that includes optical signal frequencies in order to suppress waveform deterioration of an optical signal due to propagation along an optical transmission line. In addition, it wavelength division multiplexing (WDM) transmission systems, which propagate multi-wavelength optical signals, in the same manner, it is necessary to reduce the amount of dispersion in the frequency band that includes optical signal frequencies (optical signal wavelengths) of the respective multi-wavelength optical signals.
To compensate dispersion that accumulates in each frequency component of an optical signal, a dispersion compensator is provided upon an optical transmission line. With a dispersion compensator, the dispersion that develops in an optical signal is compensated by applying an appropriate phase shift to an optical signal. Such an optical compensator is disclosed in, for example, Document 1 [The Journal of the Institute of Electronics, Information and Communication Engineers (Shingaku Gihou) Vol. 100, No. 379, OCS2000-61], Document 2 [O plus E Vol. 22, No. 9, p. 1151], and Document 3 [OFC2000, Shirasaki, et al., Variable Dispersion Compensator Using The Virtually Imaged Phased Array (VIPA) for 40-Gbit/s WDM Transmission Systems].
In recent years, due to the expansion of optical transmission systems, dispersion compensation with even higher accuracy is possible in dispersion compensators configured upon optical transmission lines, and there are also demands for dispersion compensators with superior controllability of dispersion compensation. In this regard, sufficient controllability and accuracy of dispersion compensation cannot be obtained with conventional dispersion compensators.
For example, the dispersion compensator disclosed in the above-mentioned Document 1 is configured comprising arrayed-waveguide grating (AWG). Phase adjustment is performed by the spatial phase filter for each frequency component of an optical signal that is input from the first slab waveguide side of the AWG, and output from the second slab waveguide on the opposite side to compensate the dispersion of an optical signal. However, with such configuration, since a spatial phase filter is used for phase adjustment of each frequency component of the optical signal, the applied phase shift is fixed so that dispersion compensation cannot be adjusted.
Furthermore, the dispersion compensator disclosed in Document 2 is configured so as to use a planar waveguide type optical circuit with a variable optical path difference with a Mach Zender interferometer (MZI) to perform dispersion compensation. However, with such configuration, the structure of the optical circuit becomes complex, and its size also becomes large (e.g., approximately 5 cm2). In addition, the response of phase adjustment is low (e.g., approximately 10 ms).
Furthermore, with the dispersion compensator disclosed in Document 3, a device that propagates an optical signal through space is used to change optical path length; however, with such configuration, the system is large, and highly accurate phase adjustment is difficult. In addition, the insertion loss into the optical fiber transmission line is large, for instance, 10 dB or greater
The present invention has come about in order to solve the problems mentioned above, and aims to provide a variable dispersion compensator, and an optical transmission system equipped therewith, which have superior controllability and accuracy of dispersion compensation while also allowing size reduction of that optical circuit.
In order to achieve such an objective, the variable dispersion compensator according to the present invention is a variable dispersion compensator, which applies a phase shift to an optical signal to compensate dispersion in the optical signal, and is characterized by comprising (1) optical splitting means, which inputs an optical signal that is to become the subject of dispersion compensation, and splits the optical signal for every frequency component within a predetermined frequency band (2) reflecting means, which reflects each of the respective frequency components that are split by the optical splitting means to apply a predetermined phase shift to each frequency component, and is configured with the reflection position for each of the respective frequency components being movable in the direction of optical signal propagation; and (3) optical combining means, which combines the frequency components reflected by the reflecting means to give a dispersion compensated optical signal, wherein the reflective means is constituted by a single reflective mirror; the single reflective mirror is a movable mirror capable of moving each of its reflective surfaces corresponding to the frequency component in the optical signal propagation direction by deforming the entire reflective surface thereof; and wherein the movable mirror, which is the single reflective mirror, is designed such that the entire reflective surface thereof is deformed by applying a moment to application portions provided respectively to the vicinity of the end portion on both sides by means of a moment application means, while fixing a fixed portion provided to the vicinity of the center thereof.
In the variable dispersion compensator mentioned above, the difference in optical path length from an optical splitting means, through a reflecting means, until an optical combining means is used to apply a predetermined phase shift to each frequency component of an optical signal. Then through the use of the reflecting means having a movable reflection position for each frequency component, the phase shift applied to each frequency component is made variable.
With such configuration, it is possible to compensate dispersion that develops in an optical signal with high accuracy. Furthermore, by adjusting the reflection position at the reflecting means relative to each frequency component, it is possible to control the dispersion compensation due to application of a phase shift. Furthermore, since dispersion compensation is controlled with only the reflecting means, it is possible to simplify the structure of the optical circuit, and accordingly, allow for the size reduction of the optical circuit
Moreover, according to the structure deforming the portion in the vicinity of the end portion of the movable mirror with a moment application meals as described above, the force necessary in application for deforming the movable mirror becomes small, and the entire reflective surface thereof can be deformed easier.
In addition, an optical transmission system according to the present invention is characterized by comprising (a) an optical transmission line, which propagates an optical signal having a frequency component within a predetermined frequency band; and (b) the variable dispersion compensator mentioned above, which is disposed at a predetermined position upon the optical transmission line and compensates dispersion that develops in the optical signal propagated through the optical transmission line.
Through this, dispersion that develops in an optical signal that propagates through an optical transmission line such as an optical fiber transmission line may be compensated with favorable controllability and high accuracy to achieve an optical transmission system that prevents waveform deterioration of an optical signal.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.